It's worthwhile reading the circuit analysis of the Quad II in Morgan Jones's book "Valve Amplifiers". He suggests replacing the cathode resistor and bypass cap. Other mods mentioned are increasing the B+ by going solid state on the power supply and rewiring the output stage to accommodate tubes other than the rare and expensive KT66.

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Hello!
Not much to modify is it?
Change the old elcos to new ones or PP caps. The old PIOs for new ones or even Russian teflons. Get NOS original valves. Experiment with different resistors types.
There are discussions about the crap SS diodes send into a circuit, why add SS rectification to a Quad?
Experiment with the feedback resistor, increase it to 2,7k for example (warning: might get unstable).
Increase the cathode R to 220R.
cheers

shifty: I can agree with that, Saratov KT66 are good value for money, more slam and "raw" sound compared to NOS GECs,, but hey, the price difference...
The GZ32 can't take more than 47uF, though. Read somewhere that the second psu elco can be three times that size, but haven't dared to test it.
A non-inductive cathode R might be nice to try...as is less feedback

The GZ34 is an evolution of the GZ32. It can handle more current with less voltage drop on it, so it is a very good remplacement for the GZ32.

You will get more output power with it, without the need to use solid state diodes.

A thing you must be aware on the QuadII is at it become too hot with time, because the 2 KT66 are too close from each other. So, if you want to use it more as 2 or 3 hours at a time, you MUST install a fan in the proximity of each amp.

I have used the Russian KT66 with good sucess in the QUAD II amps.... One main reason is the low plate voltage of the QUAD is kinder on these new production tubes....
I used the Groove Tubes KT66-HP and tried various strengths to measure agianst an original as well as figure which one sounds best in the QUAD II... It happens to get the #8 ratting from Groove Tubes for the KT66-HP for best Hi-Fi results....